Recently, as one example of a cooling and heating system, a district cooling and heating system using incineration plants or combined cycle cogeneration is being invigorated. The district cooling and heating system is an economical system in which a concentrated heat source plant (e.g. a cogeneration plant) supplies heat for heating, supplying hot-water or cooling to residential, commercial, government etc. buildings that request it in a city or predetermined district by means of a piping network, without using individual heat generation equipment (an oil or gas boiler, etc.). In such a district cooling and heating system, the supply of heat is embodied by the following method. First, a district cooling and heating medium which is made in the heat source plant is supplied to a heat exchanger of demanding buildings over an adiabatic pipe network. The heat medium supplied to the heat exchanger transfers heat to an internal circulation heat medium of the demanding buildings by a heat exchanger for the demanding buildings and then returns it to the heat source plant. Circulation water of the demanding buildings that has received heat in the heat exchanger chamber is supplied to each household or each floor of the buildings. The heat medium that is used in the majority of cases for district cooling and heating is water. Because of the characteristics of district cooling and heating, water is typically heated to a medium-high temperature (100 degrees or more) before circulating it through the pipe network.
In such a cooling and heating system, pipe water in a circulation piping system repetitively expands and contracts depending on variations in the temperature. If the pipe water rapidly expands, the pipe pressure sharply increases, creating such risks as the bursting of a pipe. To avoid such a risk from being caused by the expansion of water in the pipe, typical cooling and heating systems are provided with an expansion tank which serves as pressure maintenance equipment for maintaining the pressure in the pipe constant in such a way that when pipe water expands, the expansion tank temporarily receives the expansion water to reduce the pipe pressure, and when pipe water contracts, the expansion water that has been received in it returns to the pipe.
FIG. 1 is a view illustrating the construction of a conventional cooling and heating system. In this conventional cooling and heating system, a heat medium (pipe water) heated by a heat source plant 1 is supplied to or returned from a cooling and heating apparatus (a load; 10a) or an adjacent heat exchanger of a demanding building by a circulation piping system 10. The cooling and heating system includes an expansion tank 130 which branches off from the circulation piping system 10 to temporarily receive the expanded heat medium and return the heat medium to the circulation piping system 10 when the heat medium contracts; an expansion pipe 20 which branches off from the circulation piping system 10 and is connected to the expansion tank 130; and a nitrogen supply unit 200 which is connected to a predetermined portion of the expansion tank 130 to supply nitrogen gas into the expansion tank 130.
The expansion tank 130 comprises a diaphragm or non-diaphragm type hollow closed pressure tank. The expansion tank 130 is provided with a tank pressure sensor PT2 and a tank-water-level sensor LT which sense the pressure and the level of expansion water in the tank. Furthermore, the expansion tank 130 is provided with a nitrogen gas supply valve S1 which controls the supply of nitrogen gas from the nitrogen supply unit 200 into the expansion tank 130, and a nitrogen gas exhaust valve S2 which controls the exhaust of nitrogen gas from the expansion tank 130.
The nitrogen supply unit 200 supplies nitrogen gas into the expansion tank 130 and includes a compressor (not shown) which compresses air to a predetermined pressure and supplies it, and a nitrogen generator (not shown) which extracts only nitrogen from the air supplied from the compressor and supplies it into the expansion tank 130.
A method of operating the cooling and heating system will be explained below. In the initial stage, the expansion tank 130 is supplied with nitrogen gas from the nitrogen supply unit 200 so that it is filled with nitrogen gas under the initial pressure of the expansion tank 130. The water level of the expansion tank 130 is maintained at the lowest water level (LWL). If the heat medium expands, the heat medium is drawn from the circulation piping system 10 into the expansion tank 130 through the expansion pipe 20. The water level of the expansion tank 130 increases to the highest water level (HWL). At this time, the tank pressure sensor PT2 senses the internal pressure of the expansion tank that has been increased by the supply of expansion water. Then, the control unit 400 opens the nitrogen gas exhaust valve S2 so that nitrogen gas is exhausted from the expansion tank until the internal pressure of the expansion tank falls within an optimal operation pressure range.
If the expansion water returns to the circulation piping system 10 again, or if, as time passes, nitrogen gas dissolves in the heat medium, and even though it does so only by a small amount, the internal pressure of the expansion tank 130 may drop below the optimal operation pressure range. When this occurs it is sensed by the tank pressure sensor PT2 so that the control unit 400 opens the nitrogen gas supply valve S1. Thereby, nitrogen gas is supplied from the nitrogen supply unit 200 into the expansion tank 130 such that the internal pressure of the expansion tank 130 is maintained within the optimal operation pressure range.
As such, depending on physical parameters (pressure, water level) that are sensed by the pipe pressure sensor PT1, the tank pressure sensor PT2 and the tank-water-level sensor LT, the conventional expansion control apparatus supplies nitrogen gas into the expansion tank 130 or discharges it therefrom, thus controlling the internal pressure of the expansion tank 130 so that the pipe pressure of the circulation piping system 10 can be maintained constant.
The expansion tank 130 must be designed so that its capacity is such that it can receive the amount of expansion water that is formed when pipe water of the circulation piping system 10 expands. However, because the capacity of the expansion tank 130 that can be designed is limited, if the circulation piping system 10 is of high capacity, it is difficult for only the single expansion tank 130 to encompass the entirety of the expansion water. Hence, multiple expansion tanks are typically used.
In the system provided with multiple expansion tanks, an additional expansion pipe is provided branching off from the existing expansion pipe 20, and an additional expansion tank is connected to the additional expansion pipe in such a way that the additional expansion tank is parallel to the existing expansion tank 130. A tank pressure sensor and a tank-water-level sensor are provided on each expansion tank.
In the pressure maintenance equipment provided with the multiple expansion tanks, the supply of gas into each expansion tank or the exhaust of gas therefrom may be individually controlled based on values measured by the corresponding tank pressure sensor and tank-water-level sensor. Alternatively, the mean of values measured by the sensors may be used to simultaneously control the supply of gas into the expansion tanks or the exhaust of gas therefrom.
However, in the case where each expansion tank is individually controlled, if an error between the sensors is comparatively large, the expansion tanks may be differently controlled despite being in the same system. For instance, despite the fact that nitrogen gas must be exhausted from the expansion tank when pipe water expands, if an error of the sensor of any expansion tank is significant or the sensor itself malfunctions, nitrogen gas may be supplied to the expansion tank in opposition to the state of the other expansion tank or the entire system.
On the other hand, in the case where only the mean of measurement values of the sensors is simply used to simultaneously control the expansion tanks, if any one of the multiple sensors is not within a permissible range or a temporary abnormality has occurred in any sensor, a difference between the actual pipe pressure of the system and a reference control value makes it difficult to appropriately control the system in response to the conditions of the system. For example, supposing that the optimal pressure of the expansion tanks is 9.0 kg/m2·G, if the pressure of an expansion tank provided with a normal sensor is measured to be 10.0 kg/m2·G while the pressure of the other expansion tank is measured to be 0 kg/m2·G because of an abnormality in the sensor, the reference control value becomes 5 kg/m2·G as a result of a simple calculation of the mean, so nitrogen gas is supplied to the expansion tanks despite the fact that nitrogen gas should be exhausted from the expansion tanks until the internal pressure of the tanks reaches 9.0 kg/m2·G.
As such, if the control of pressure of the system cannot respond to the actual conditions of the system because of an error and malfunction of any sensor, not only may the pressure maintenance equipment itself malfunction, but it may also have a great influence on the entirety of the cooling and heating system. That is, if the internal pressure of the expansion tanks excessively decreases, flushing occurs in the pipe, causing a big accident or defective heating. If the internal pressure of the expansion tanks excessively increases, excessive pressure is applied to the piping equipment, thus damaging the pipes or other equipment.